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The Cape Peninsula University of Technology (CPUT) Electronic Theses and Dissertations (ETD) repository holds full-text theses and dissertations submitted for higher degrees at the University (including submissions from former Cape Technikon and Peninsula Technikon).

An analogue controlled switch-mode power system for a CubeSat

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The power system is essentially one of the most critical subsystems to any satellite, without
some form of power system a satellite would simply cease to function. The research within these
pages investigates the areas pertaining to satellite power systems with the main focus towards
the CubeSat platform. The end objective of this research was the development of a reliable
analogue based switch-mode power system for a CubeSat.
The research began with an investigation into the CubeSat platform, the space environment
and a basic overview of a satellite and its systems. The research then focussed on satellite
power systems, focussing primarily on satellite power system topologies and switch-mode
power systems. Various components and concepts surrounding the satellite power system were
investigate and included the photovoltaic (PV) solar cell, batteries, satellite power system
topologies, protection concepts and typical CubeSat power systems. The nal part of the
literature review included research into typical CubeSat power systems.
The space environment complicates the design of satellite systems. The developed electrical
power system harnessed electrical power from a PV solar panel by means of a fractional opencircuit
voltage (FOCV) based maximum power point tracker (MPPT) with the use of a SEPIC
DC-DC converter. The use of a SEPIC DC-DC converter allowed the system to operate at a
greater e ciency than could be expected from linear designs. The requirement for an e cient
system was important as the heat generated by the power system could bring rise to dissipation
issues, resulting in over-heating of various components. The design took into account component
sizing, as larger components would be more prone to damage during the high accelerations and
vibrations associated with being launched into space. The use of a MPPT allowed the power
system to better utilise the available PV solar panel power, by maintaining the PV solar panel
near its optimum operating voltage. The design slid between MPPT and voltage regulation to
harness as much power as possible while not over-charging the Lithium polymer battery. The
power system consisted of battery under-voltage protection as well as over-current protection for
the attached payloads and satellite subsystems.
The SEPIC DC-DC converter was selected over other SMPS topologies, as this topology could
be used in a 1U and 3U CubeSat with a wide variety of PV solar panel cell con gurations. The
bene ts of this SMPS topology are due to the SEPIC DC-DC converter's ability to produce an
output voltage greater than, less than or equal to the input voltage (National Semiconductor,
2008; Texas Instruments, 2008a). This, along with the operation of the FOCV based MPPT,
allowed the power system to be very exible. The designed FOCV based MPPT could be pre-set to di ering PV solar cell technologies due to the adjustable ratio between the maximum power
point voltage, Vmpp, and the open-circuit voltage, Voc of the PV solar panel. It was decided not
to select a Buck or Boost DC-DC converter based power system as this would limit the exibility
of the system. Additionally, the SEPIC DC-DC converter brings with it the ability to isolate the
input and output voltage upon shut down. This isolation is due to the SEPIC DC-DC converter's
coupling capacitor and this topologies operation as described by National Semiconductor (2008)
and Texas Instruments (2008a).
The prototype was versatile allowing a wide variety of PV solar cell technologies to be used.
The wide operating voltage of the prototype allowed the design to be connected to a series or
parallel combination of solar cells with an operating voltage of 3 V to 20 V. The power handling
capability of the prototype per solar panel channel allows the design to be applied to a 1 U
or 3 U CubeSat given that the channel did not exceed 10 W. All components of the prototype
operated without fault, e ectively charging the Li-poly battery safely while protecting payloads
and subsystems. The SEPIC DC-DC converter utilised by the MPPT achieved an e ciency of
71 % under full load and with an input voltage of 10 V.

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